1.1 Radio positioning
Radio positioning technology can be subdivided into categories such as WIFI, GSM/LTE, Blue tooth, ZIGBEE, UWB, millimeter wave radar, etc. based on the carrier radio [4]-[6]. WIFI wireless local area network positioning has the advantages of low deployment cost and integration with communication system, but the tag power consumption is high; the GSM/LTE uses underground mobile communication network to realize mobile phone positioning, since the underground mobile communication base station has less cross coverage, the positioning accuracy is low; Blue tooth is characterized by low power consumption and short coverage distance. In most situation, The underground blue tooth beacon was used to achieve regional positioning; ZIGBEE has been mainstream technology for current underground personnel positioning system, with the characteristics of low power consumption and short distance, it can achieve meter-level precision; UWB has the advantages of large capacity, low power consumption, and centimeter-level accuracy which is currently dominant the high-precision positioning application; millimeter-wave radar has millimeter-level accuracy which is generally used for obstacle recognition and accurate ranging of underground mobile equipment.
Based on the positioning principle of radio technology, the mainstream methods include RSSI( Receiving Signal Strength Indication, RSSI), AOA (Angle Of Arrival, AOA), PDOA (Phase Difference of Arrival, PDOA), TOA (Time Of Arrival, TOA), TDOA (Time Difference Of Arrival, TDOA) and other methods [7]-[8]. RSSI calculates the distance between the mobile node and the anchor node based on the radio signal transmission loss model. In underground mines, the radio signal transmission loss model is greatly affected by the environment which results in more than 10 meters positioning accuracy; the AOA method measures the angle of the target node, this method generally requires an integrated antenna array, which is difficult to apply in underground mines; the PDOA method measures the phase difference of the arrival of the radio signal and calculates the distance to the target node. This method is susceptible to interference from multipath effects; the TOA method measures the propagation delay of radio signals and uses the constant speed of light to calculate the distance, the coordinates can be calculated based on distances from the tag to more than three anchors using trilateral least squares and other algorithms. TOA related methods are simple and easy to implement, but the round-trip measurement between nodes increases the congestion of the radio transmission and reduces the number of concurrency and real-time performance of the positioning system; TDOA calculates the position of the mobile node by measuring the difference in the arrival time of the radio signal from the tag to several anchors, This method requires highly consistent clock synchronization of base stations, TDOA has high positioning accuracy, large system capacity, and short response time, but the technology is relatively complex and high-cost to implement in underground mine.
1.2 Geomagnetic positioning
Geomagnetic positioning collects geomagnetic characteristic parameters at any locations in the space to be measured and establishes a characteristic database [9]. In actual positioning, the parameters are collected by sensors and matched and retrieved with the geomagnetic data collection in the database. Ideally, the geomagnetic positioning accuracy can reach about 2 meters.
The advantage of geomagnetic positioning is that there is no need for additional infrastructure support such as positioning base stations, and only the tag needs to support geomagnetic feature collection, which is relatively low cost. To achieve ideal geomagnetic positioning performance, geomagnetic characteristic parameters from all physical spaces on the site need to be collected in database. Once the characteristics change, the parameter records need to be updated again, which make it hard and complex to implement on site.
The underground mine is a special place with a relatively complex environment. Electromagnetic interference of various electromechanical equipment, metal shielding and reflection of mining equipment and protective nets, etc. make the underground geomagnetic parameters extremely unstable, and even unable to effectively extract characteristic parameters. It’s difficult to rely only on geomagnetic information to achieve accurate position in underground mine.
1.3 Inertial Positioning
Inertial positioning uses IMU (Inertial Measurement Unit, IMU) such as multi-axis acceleration sensors, gyroscopes, and magnetometers to measure the directional velocity and acceleration of the measured tag in three-dimensional space, and calculate the tag position according to certain rules [10]. The advantage of inertial positioning is that it does not require the support of additional hardware infrastructure, but it has the problem of starting position drift, and small errors in the measurement will cause big positioning estimation error over time.
Because of the high cost, high-precision inertial positioning technologies such as lasers and optical fibers are only used in a few scenarios such as positioning for shearer. For most underground positioning applications, MEMS (Micro-Electro-Mechanical System, MEMS) inertial positioning technology is used. MEMS inertial sensors have the advantages of small size and low power consumption, but generally have low accuracy, especially temperature changes can easily cause measurement and calculation errors to accumulate, causing speed and position to diverge, which has big impact on the final positioning accuracy.
1.4 Visual Positioning
Visual positioning uses computer image processing technology to estimate the distance and position of the target to be measured from the digital image. Visual positioning can be divided into monocular visual positioning, binocular visual positioning and RGB-D visual positioning [11]. Monocular visual positioning has low cost and poor accuracy; binocular visual positioning are more accurate depending on more complicated calculations; RGB-D has even better performance which can realize three-dimensional position calculations and avoid light effects.
Visual positioning has been partially applied in underground mines, such as underground video surveillance, Intrusion into dangerous area monitoring. Due to the limitation of the capturing range of camera, the visual positioning is only suitable for the target positioning in narrow directional and small range area in underground mine, and is not suitable for the whole mine positioning with large capacity and multiple targets. The positioning accuracy of visual positioning can generally reach the meter level, but due to the influence of underground mining dust, the accuracy and reliability of the actual scene still have space to be improved.
1.5 Comparison of positioning technologies
Comprehensive comparison and analysis of the advantages and disadvantages of various underground positioning technologies, as the following Table 1, among different underground positioning methods, UWB has obvious advantages, and each method has certain shortcomings. From the perspective of the application requirements of MLBS, it’s difficult for a single technology to meet all the needs. Combining the advantages of one or more technologies especially the fusion method can further improve the positioning accuracy and reliability in the complex underground environment.
Table 1 Comparison of performances of underground positioning methods
Mine Positioning Technology
|
Accuracy
|
Distance
|
Price
|
Advantages
|
Disadvantages
|
Radio
Positioning
|
WIFI
|
3m-10m
|
100m-600m
|
Low
|
Mature technology and easy to deploy
|
Low accuracy and high power consumption
|
GSM/LTE
|
25m-50m
|
100m-400m
|
High
|
Integration of positioning and communication
|
Low accuracy and high terminal price
|
Blue Tooth
|
1m-3m
|
3m-15m
|
Low
|
Ultra-low power consumption
|
Small coverage
|
ZIGBEE
|
1m-3m
|
30m-250m
|
Low
|
Low power consumption and easy to deploy
|
Unreliable accuracy
|
UWB
|
0.1m-0.3m
|
200m-800m
|
Low
|
High accuracy and Ultra-low power consumption
|
NLOS issue
|
Millimeter Wave Radar
|
0.02m-0.05m
|
20m-100m
|
Medium
|
High accuracy
|
Small coverage
|
Geomagnetic Positioning
|
1m-3m
|
--
|
Low
|
No need base station
|
Unstable positioning feature
|
MEMS IMU
|
0.2m-1m
|
--
|
Low
|
No need base station
|
Error accumulation
|
Visual Positioning
|
1m-3m
|
10m-50m
|
High
|
Integration of positioning and video surveillance
|
Limited application scenarios
|
1.6 Current Status and application requirements
Most of the positioning systems currently used in mines are designed for single application and only meet partial services. With the further development of intelligent coal mines, the demand for less humanized and unmanned underground operations has become increasingly obvious. At the same time, higher performance requirements such as accuracy, capacity and response time have been put forward for the underground positioning system. Some problems of the existing positioning system include the following:
(1) Insufficient positioning accuracy, capacity and real-time performance. The current positioning accuracy is generally less than 3 meters. The concurrent capacity of a single base station is 80 tags. The positioning interval is generally greater than 1.5 seconds, which cannot be satisfied the requirements of large-capacity nodes, fast movement and high-precision positioning.
(2) The centralized system structure cannot meet the demand of real-time response. The server on the surface is the center of the existing system which collects all the positioning data and implements calculations, the tags cannot sense the location information directly, which makes it impossible to respond immediately for some mine application scenarios such as personnel vehicle collision avoiding and collaborative mining operation.
(3) Only supports one-dimensional positioning. The mining equipment not only moves in the horizontal direction, but also has a multi-degree-of-freedom movement. It is important for mining equipment to sense accurate position of each other while collaboratively mining, two-dimensional and three-dimensional positioning service will be necessary.
(4) Does not have open service application interface. Different underground applications have different functional requirements for the accuracy, dimension, and response time. Lack of unified access standards and universal application interfaces which enables multiple application systems requirements to be satisfied are becoming the bottleneck of positioning system development.
Therefore, the construction goal of the high-precision MLBS should be set as the public infrastructure of intelligent mines. Through the establishment of an open service and unified architecture underground GPS-like system, it can meet the positioning, tracking, navigation and coordination of different scenarios and different application requirements in underground mines.